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Cracking the mystery of not cracking the eggshell

Greater Ani is not your regular cuckoo. Rather than dump its eggs in the nests of other species it builds its own and raises its young alongside other breeding Anis. The eggs bump into each other with quite some force as parent birds turn them during incubation. Now, researchers have shown that Anis have evolved eggs with the added protection of an uncommon mineral that keeps the eggs from cracking under pressure.

Vaterite, a form of calcium carbonate, sits on the eggshell of some birds in tiny spheres of varying size. It is less stable and abundant than calcite – another form of calcium carbonate that primarily constitutes the shell. Vaterite gives freshly laid Ani eggs their white chalky appearance. As incubation progresses, it gets scratched, exposing patches of the pigmented calcite underneath. When scientists studying nesting Greater Anis (Crotophaga major) in the Panama Canal noticed this change in the egg surface they decided to look deep into it.